Power transformer of the symmetric-asymmetric type with a fully-balanced topology

ABSTRACT

A transformer of the symmetric-asymmetric type includes comprising a primary inductive circuit and a secondary inductive circuit formed in a same plane by respective interleaved and stacked metal tracks. A first crossing region includes a pair of connection plates facing one another, with each connection plate having a rectangular shape that is wider than the metal tracks, and diagonally connected to tracks of the secondary inductive circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 16/275,091, filed Feb. 13, 2019, which is a continuationapplication of U.S. patent application Ser. No. 15/223,148, filed Jul.29, 2016, now issued as U.S. Pat. No. 10, 249,427, which is atranslation of and claims the priority benefit of French patentapplication number 1652713, filed on Mar. 30, 2016, and entitled“Balanced-To-Unbalanced Transformer For Power Application With FullyBalanced Topology,” which are hereby incorporated by reference herein intheir entirety.

TECHNICAL FIELD

The disclosure relates to integrated transformers of thesymmetric-asymmetric type, commonly denoted by the term BALUN (BALancedto UNbalanced). The disclosure is, for example, applicable in mobiletelephony and motor vehicle radar systems.

BACKGROUND

The fabrication of integrated systems made of silicon, whether power orprocessing systems, is increasingly implemented using differentialstructures and variable reference impedances for analog parts. On theother hand, most everything else remains essentially a system of theasymmetric mode type with 5 o Ohms reference impedances.

The link between a symmetric transmission line and an asymmetrictransmission line cannot be implemented without a matched electricalcircuit. This transition is provided by a transformer of thesymmetric-asymmetric type called a balun.

A balun converts, for example, a signal of the asymmetric mode type (orsingle-ended according to terminology widely used by those skilled inthe art) into a signal of the differential mode type, and vice-versa.The balun also ensures the transformation of impedances.

One of the main electrical characteristics of a balun is its insertionloss, which is advantageously as low as possible. Indeed, the insertionloss is the result in loss of the transformation applied. The loss maybe due to an impedance mismatch, an imbalance in amplitude and/or phasebetween the two channels, a resistive loss, and/or all of these factorscombined. This loss causes a reduction in the overall performance of thesystem employing this device.

Furthermore, the performance characteristics of a balun are mainlyexpressed in terms of amplitude and phase symmetries. There is adifference in amplitude and a phase shift between the input and outputsignals which are advantageously minimized.

Baluns may furthermore be used, for example, in receiver andtransmission circuits of wireless communications systems. In particular,for the design of differential circuits such as amplifiers, mixers,oscillators and antenna systems.

In the transmission and receiver circuits of wireless communicationssystems, the impedance on the differential side may be low, typically onthe order of 10 to 20 Ohms for a low-noise amplifier. The impedance onthe single-ended side, in other words on the side of the antenna, asindicated above, is generally around 50 Ohms. This, therefore, meansthat a high transformation ratio is necessary, which can be particularlycomplicated to achieve.

Furthermore, notably in transmission, the power amplifier is to besupplied with a high current, on the order of a few hundred milliamps.Then, if it is desired to supply the power amplifier by means of thetransformer (balun), this will have an impact on the performance of thebalun.

For example, the high currents require a very wide metal track, whichintroduces an increase of the series resistance which is to thedetriment of the insertion loss. Consequently, the design of baluns isusually limited to one turn per loop on the secondary circuit forhigh-power circuits. This has the consequence that the coupling betweenthe differential and single-ended channels is generally unequal andpoorly distributed. This leads to poor performance characteristics, suchas phase-shifts and amplitude mismatches.

SUMMARY

According to one embodiment, an integrated architecture is provided fora transformer of the symmetric-asymmetric type that is totally balanced,which allows signals to be obtained that are in phase and withcorresponding amplitudes. This may notably be for power amplifierapplications.

A transformer of the symmetric-asymmetric type may comprise an inductiveprimary circuit and an inductive secondary circuit formed in the sameplane by respective interleaved and stacked metal tracks. The tracks maycomprise at least a first crossing region in which two connection platesfacing one another take the form of rectangular plates, wider than themetal tracks, and may each be diagonally connected to tracks of thesecondary circuit.

The plane shapes facing one another of the crossing regions may offer alarge crossing surface area. This may increase the coupling capacitancebetween all the turns of the transformer. Advantageously, notably inregards to noise signals, the widened portions may be the same size andmay be aligned along an axis perpendicular to the plane.

The connection plate passing over the other connection plate maycomprise two wings each respectively situated on one end of two opposingsides of the rectangular plate. The ends may be diagonally opposite andthe metal tracks of the secondary circuit may be connected to the lowersurface of the wings. Advantageously, the wings may each have a bevel atits connection with the rectangular plate. This configuration is notablyadvantageous in regards to current flow, such as in the case of a highintensity current flow.

The primary and secondary inductive circuits may each comprise a loopdescribing at least two turns and have an architecture that issymmetrical with respect to an axis of the plane. A geometricallysymmetrical and balanced architecture with respect to coupling minimizesor reduces the phase and amplitude imbalances of the signals present onthe primary and secondary circuits.

Generally speaking, one terminal of the primary circuit may be connectedto a load and the other terminal to ground. Consequently, the couplingbetween the primary and secondary circuits does not take place in thesame way between the tracks at positions close to the load terminal andat positions close to the ground terminal.

The primary and secondary inductive circuits may be configured suchthat, over all of the positions of the secondary circuit at which acoupling with the primary circuit may take place, the sum of thedistances from one terminal of the primary circuit to the correspondingcoupled positions of the primary circuit may be equal to the sum of thedistances from the other terminal of the primary circuit to the samecoupled positions.

In this configuration, the secondary circuit may be coupled with theprimary circuit in equal proportions at positions of the primary circuitclose to one terminal and at positions of the primary circuit close tothe other terminal. In other words, the signal in the secondary circuitsees the ground terminal as much as the load terminal of the primarycircuit.

Thus, during the flow of a signal over the secondary circuit, thissignal may be coupled in a uniform manner with the whole of the primarycircuit, offering good phase and amplitude symmetries. This allowsexcellent behaviors to be obtained with regard to the balance of phasesand the balance of amplitudes. This is notable for power amplifierapplications.

In one embodiment, the at least a first crossing region may comprisesfirst metal tracks for connecting tracks of the primary circuit crossingeach other under the connection plates.

According to one embodiment, the interleaved loops may comprise at leasta second crossing region, in which second metal tracks for connectingthe primary circuit cross each other on either side of a biasingterminal. One of the second connection tracks may pass above the biasingterminal and the other underneath.

Thus, the symmetry of the architecture and the balance of the couplingsbetween the primary and secondary circuits may also be optimized at thecrossing regions. Advantageously, the biasing terminal takes the form ofa rectangular plate connected to a mid-point of the secondary circuitand is situated in the neighborhood of the terminals of the secondarycircuit. This allows decoupling capacitors to be connected between thebiasing terminal and the ground of the differential circuit in anoptimized manner with regard to space and performance.

According to one embodiment, the metal tracks of the primary circuit maybe narrower than the metal tracks of the secondary circuit, over atleast a portion of the primary circuit. This may allow, aside from anadvantageous reduction in the surface area occupied by the transformer,a stray capacitance between the primary circuit and ground of thesubstrate on which the transformer is fabricated to be limited.

According to one embodiment, the transformer may be fabricated in anintegrated manner on top of a semiconductor substrate.

A circuit may also be provided that comprises an antenna, processor orprocessing circuit and a transformer previously described, connectedbetween the antenna and the processor. Furthermore, a telecommunicationssystem may be provided, for example, of the cellular mobile telephonetype, or tablet or equivalent, comprising such a circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features of the disclosure will become apparentfrom examining the detailed description of embodiments and theirimplementation, which are in no way limiting, and from the appendeddrawings in which:

FIG. 1 shows a transformer according to the disclosure in a plan view;

FIGS. 2 and 3 show the crossing regions of the transformer inperspective views;

FIG. 4 shows an input or output stage of a radio frequencytelecommunications system comprising a transformer according to thedisclosure.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 shows a view from above of one embodiment of asymmetric-asymmetric transformer, or balun BLN. The balun BLN belongs toa plane P comprising an axis X forming an axis of symmetry for the wholearchitecture of this embodiment, and is fabricated on a semiconductorsubstrate SC.

The balun BLN comprises a primary inductive circuit L1 formed by metaltracks whose disposition forms an octagonal loop, which is wound andunwound while making three complete rotations, or three turns. Theprimary circuit L1 comprises two terminals SE and GND designed to beconnected in asymmetric, or single-ended, mode respectively to a loadand to ground. For example, the load may be a transmitting or receivingantenna.

The terminals SE and GND of the primary circuit L1 are disposedside-by-side in a symmetrical manner with respect to the axis X, on anexternal side of the balun BLN. The balun BLN also comprises a secondaryinductive circuit L2, formed by metal tracks whose disposition forms anoctagonal loop which is wound and unwound while making two turns, in aninterleaved manner with the turns of the loop of the primary circuit L1.

The metal tracks P11-P15, P21-P25 forming the turns of the primary L1and secondary L2 circuits are situated in the same metallization level.Furthermore, the octagonal geometries of the loops of the primary andsecondary circuits are given by way of a non-limiting example, and maytake another polygonal or circular form.

The secondary circuit L2 comprises two terminals PA1 and PA2 designed tobe connected in a symmetric, or differential, mode to transistors of apower amplifier circuit, for example. A biasing terminal VCC isconnected to a mid-point of the secondary circuit L2 and is designed toreceive a common-mode DC voltage.

The terminals PA1, VCC and PA2 of the secondary circuit L2 arerespectively disposed side-by-side in a symmetrical manner with respectto the axis X. This is on an external side of the balun BLN, opposite tothe side comprising the terminals SE, GND of the primary circuit L1.

Thus, the interleaved nature of the primary L1 and secondary L2inductive circuits provides an arrangement in which the metal tracks ofthe turns of the primary circuit L1 are disposed on either side of, anddirectly next to, the track of each turn of the secondary circuit L2.The winding and unwinding of the turns of the primary and secondarycircuits introduce crossing points for metal tracks. Thus, the metaltracks are stacked, notably in the crossing regions, passing over andunder the metallization level of the turns, in respectively higher andlower levels of metallization.

It is nevertheless considered that the balun BLN is included within aplane P and that the symmetry with respect to the axis X does not takeinto account the differences in height of the levels of metallization.This is commonly admitted in microelectronics due to the very smallvertical dimensions of the architecture.

Thus, the balun BLN comprises two crossing regions CR1 and CR2 in whichthe metal tracks cross one another, via metal tracks referred to asconnection tracks.

The first crossing region CR1 is situated in the turns on the side ofthe terminals SE, GND of the primary circuit and comprises a crossing ofthe primary circuit L1 and a crossing of the secondary circuit L2. Thesecond crossing region CR2 is situated in the turns on the side of theterminals of the secondary circuit L2 and comprises a crossing of theprimary circuit, passing vertically on either side of the biasingterminal VCC.

The primary circuit L1 runs from the terminal SE to the terminal GND viaa track P11 which arrives at the second crossing region CR2. A metalconnection track PL6 directs the turn towards the interior of the loopand connects the track P11 to a track P23 which runs to the firstcrossing region CR1. In the crossing region CR1, a connection track PL4directs the turn towards the interior and connects the track P23 to atrack P15.

The primary circuit L1 has described a first turn (one completecircuit). The circuit then describes a second turn according to twohalf-turns formed by the tracks P15 and P25 connected together at amid-point. The loop of the primary circuit has so far been wound andthen starts to unwind. The track P25 arrives at the first crossingregion CR1, in which a connection track PL3 directs the turn towards theexterior and connects the track P25 to a track P13. The track P13 runsto the second crossing region CR2, in which the connection track PL5directs the turn towards the exterior and connects the track P13 to atrack P21. The track P21 then arrives at the ground terminal GND. Thetracks of the primary circuit L1 have thus formed a loop of three turnswhich is wound and unwound.

The secondary circuit runs from the terminal PA1 to the terminal PA2passing under the track P11 to join with a track P12 which arrives atthe first crossing region CR1. In the crossing region CR1, a connectionplate PIA directs the turn towards the interior and connects the trackP12 to a track P24. The track P24 follows a half-turn up to a mid-pointposition connected to the biasing terminal VCC. Here, the secondarycircuit L2 has formed a first turn by winding and starts to unwind. Atrack P14 starts from the mid-point and arrives at the first crossingregion CR1 in which a connection plate PL2 directs the turn towards theexterior and connects the track P14 to a track P22. The track P22arrives at the terminal PA2 after passing under the track P21.

The tracks of the secondary circuit are disposed between the tracks ofthe primary circuit. In particular, the track P12 is situated betweenthe track P11 and P13, the track P14 is situated between the tracks P13and P15, the track P22 is situated between the track P21 and P23, andthe track P24 is situated between the tracks P23 and P25. A constant gapseparates, from edge to edge, the tracks of the primary circuit and thetracks of the secondary circuit.

Such a configuration forms a structure such that, over all of thepositions of the secondary circuit at which a coupling with the primarycircuit takes place, the sum of the distances from one terminal of theprimary circuit to the corresponding coupled positions of the primarycircuit is equal to the sum of the distances from the other terminal ofthe primary circuit to the same coupled positions.

In this configuration, the secondary circuit is coupled with the primarycircuit in equal proportions at positions of the primary circuit closeto one terminal and positions of the primary circuit close to the otherterminal. In other words, the signal on the secondary circuit sees theground terminal GND as much as the load terminal SE of the primarycircuit.

Thus, when a signal travels over the secondary circuit, this signal iscoupled in a uniform manner with the whole of the primary circuit,providing good phase and amplitude symmetries. This allows excellentbehaviors with regard to balance of phases and balance of amplitudes tobe obtained, and notably for power amplifier applications.

Moreover, the tracks P11, P21, P15 and P25 of the primary circuit L1 arenarrower than the other tracks. Their width is approximately half of thewidth of a track of the secondary circuit L2. Narrower metal tracksnotably allow the stray capacitance existing between the metal tracksand the substrate to be reduced. The current flowing in the primarycircuit is usually lower than that flowing in the secondary circuit.Thus, an advantageous decrease in the width of the tracks over certainparts of the primary circuit is not detrimental with respect to currentflow.

It is also possible to form each of the tracks P13 and P23 in the formof two narrow parallel tracks. Each narrow parallel track may beseparated from the edge of the tracks of the secondary circuit by thesame constant separation. In this embodiment, the tracks for connectingthe primary circuit can have the same thickness as the tracks of thesecondary circuit. This is advantageous with regard to noise signals.

FIG. 2 shows a perspective view of the first crossing region CR1 inwhich the interleaved and stacked metal tracks are shown in transparencyfor a better understanding of the architecture of this embodiment. Inthe first crossing region CR1, the metal track of the secondary circuitP14 is connected to the metal track P22 via a connection plate PL2. Themetal track of the secondary circuit P24 is connected to the track P12via another connection plate PL1.

The connection plate PL2 is formed at the same level of metal as themetal tracks forming the turns of the primary and secondary inductivecircuits, and takes the form of a rectangular plate. The tracks P14 andP22 are connected to the connection plate PL2 on two opposing sides ofthe rectangular plate, each on one respective end of the side, with theends being diagonally opposite.

The connection plate PL1 is formed on a level of metal that is higherthan the level of the metal tracks of the primary and secondaryinductive circuits. The connection plate PL1 also takes the form of arectangular plate additionally comprising two wings respectively on twoopposing sides of the rectangular plate. Each wing is on one end of therespective side, and with the ends being diagonally opposite.

The tracks P12 and P24 are connected to the connection plate PIA on thelower surface of the respective wings. Furthermore, the connectionplates PIA and PL2 are the same size and are aligned in a vertical axisperpendicular to the plane. The diagonals along which the tracks of thesecondary circuit are connected to one connection plate or anotheropposite to each other.

Moreover, in this non-limiting representation, the wings of theconnection plate PIA each have a bevel 1 and 2 at their attachment withthe rectangular plate PIA. This configuration is advantageous withregard to current flow and is not detrimental to the balanced aspect ofthe couplings implemented by the disclosure. Indeed, although not beingstrictly geometrically symmetric with respect to the axis X, thisconfiguration is balanced with regard to coupling between the primaryand secondary circuits.

FIG. 3 shows a perspective view of the second crossing region CR2 inwhich the interleaved and stacked metal tracks are also shown intransparency for a better understanding of the architecture provided forthis embodiment. In the second crossing region CR2, the metal track ofthe primary circuit PAA is connected to the metal track P23 via aconnection track PL6. The connection track PL6 is at a lower level thanthe metallization level of the tracks forming the turns of the circuit,passing under the biasing terminal VCC.

The metal track P13 is connected to the track P21 via a connection trackPL5, passing over the biasing terminal VCC, in a higher metallizationlevel than the metallization level of the tracks forming the turns ofthe circuit.

The biasing terminal VCC takes the form of a rectangular plate and isconnected along one of its widths in such a manner as to be centered onthe mid-point of the secondary circuit. The width of the rectangularplate of the biasing terminal measures around twice the width of a metaltrack due to the high current flowing on the biasing terminal.

Thus, the connection tracks PL5 and PL6 cross each other on either sideof the biasing terminal VCC in a symmetrical manner with respect to theaxis X. This provides good performance characteristics with regard tophase and amplitude symmetries.

The connection tracks PL5 and PL6 may take the form of rectangularplates of identical size to the plate of the biasing terminal VCC,superposed over each other and with the biasing terminal. All three arealigned along a vertical axis perpendicular to the plane P.

The disclosure may advantageously be employed for any power applicationin radio frequency (RF) telecommunications systems, and FIG. 4 shows oneexample of an input or output stage of such a system SYS. For example,the system is of the cellular mobile telephone or tablet type, andcomprises a balun BLN according to the disclosure.

The terminal SE of the primary circuit L1 of the balun BLN is connectedto an antenna ANT, typically with an impedance of 5 o Ohms, and theterminal GND is connected to an external ground. The antenna may be usedboth as a transmitter and a receiver.

The terminals PA1 and PA2 of the secondary circuit L2 are, on the otherhand, connected to processing circuit or a processor MTD in differentialmode. This may comprise, for example, a low-noise amplifier LNA. Themid-point of the secondary circuit L2 is connected to a decouplingcapacitor Cap connected to the ground GND PA associated with thedifferential-mode circuit connected to the terminals of the secondarycircuit L2.

The balun BLN thus supplies an output signal in a differential mode (orin single-ended mode) starting from an input signal received in asingle-ended mode (or in differential mode) with very little losses,excellent phase and amplitude symmetries, while at the same timeallowing the passage of a current of high intensity. Such performancecharacteristics allow the efficiency of power amplifiers combined withthe transformer BLN according to the disclosure to be optimized.

Furthermore, the disclosure is not limited to the embodiments that havejust been described but encompasses all their variations. Thus, a baluncomprising a primary circuit with three turns and a secondary circuitwith two turns has been described, but it is possible, notably in orderto design the impedance transformation ratio of the balun BLN, for theprimary circuit to comprise N+1 turns and the secondary circuit tocomprise N turns. N is an integer number greater than or equal to 2. Thenumber of first crossing regions and of second crossing regionscomprising the features previously described may vary as a function ofthe number of turns on the primary and secondary circuits.

What is claimed is:
 1. A balun transformer, comprising: a primaryinductive circuit having a first terminal and a second terminal locatedside-by-side and symmetrically to a first axis and on a first side ofthe balun transformer; and a secondary inductive circuit having a thirdterminal, a fourth terminal, a fifth terminal, and at least two loops,the secondary inductive circuit located at a second side of the baluntransformer opposite the first side, wherein the primary inductivecircuit and the secondary inductive circuit are disposed on a plane,wherein the primary inductive circuit includes more loops than thesecondary inductive circuit, and wherein the third terminal and thefourth terminal are located symmetrically to the first axis, the firstaxis crossing the fifth terminal symmetrically.
 2. The balun transformerof claim 1, wherein the fifth terminal is connected to a midpoint of thesecondary inductive circuit.
 3. The balun transformer of claim 1,wherein the secondary inductive circuit comprises two loops between thethird and fourth terminals, and wherein the primary inductive circuitcomprises three loops between the first terminal and the secondterminal.
 4. The balun transformer of claim 1, wherein the primaryinductive circuit has an octagonal shape.
 5. The balun transformer ofclaim 1, wherein the secondary inductive circuit comprises a pluralityof tracks, and wherein tracks of the primary inductive circuit arelocated next to and at opposite sides of the plurality of tracks of thesecondary inductive circuit.
 6. The balun transformer of claim 1,wherein the primary inductive circuit comprises a first connection trackand a second connection track, wherein the first connection track islocated below the fifth terminal, and wherein the second connectiontrack is located above the fifth terminal.
 7. The balun transformer ofclaim 6, wherein the first connection track, the second connectiontrack, and the fifth terminal are substantially identical in shape andare aligned along a vertical axis perpendicular to the plane.
 8. Thebalun transformer of claim 7, wherein the first connection track, thesecond connection track, and the fifth terminal have a rectangularshape.
 9. The balun transformer of claim 1, wherein the secondaryinductive circuit comprises a plurality of tracks, and wherein at leastone track of the primary inductive circuit is narrower than theplurality of tracks of the secondary inductive circuit.
 10. A baluntransformer, comprising: a primary inductive circuit having a firstterminal and a second terminal located side-by-side and symmetrically toa first axis; and a secondary inductive circuit having a third terminal,a fourth terminal, and a fifth terminal, the fifth terminal connected toa midpoint of the secondary inductive circuit, wherein the primaryinductive circuit and the secondary inductive circuit are disposed on aplane, wherein the secondary inductive circuit comprises at least twoloops, and wherein the primary inductive circuit comprises more loopsthan the secondary inductive circuit.
 11. The balun transformer of claim10, wherein the first terminal and the second terminal are located on afirst side of the balun transformer, and wherein the third terminal, thefourth terminal, and the fifth terminal are located on a second side ofthe balun transformer opposite the first side.
 12. The balun transformerof claim 10, wherein the secondary inductive circuit comprises two loopsbetween the third terminal and the fourth terminal, and wherein theprimary inductive circuit comprises three loops between the firstterminal and the second terminal.
 13. The balun transformer of claim 10,wherein the secondary inductive circuit comprises a plurality of tracks,and wherein tracks of the primary inductive circuit are located next toand at opposite sides of the plurality of tracks of the secondaryinductive circuit.
 14. The balun transformer of claim 10, wherein thesecondary inductive circuit comprises a plurality of tracks, and whereinat least one track of the primary inductive circuit is narrower than theplurality of tracks of the secondary inductive circuit.
 15. A baluntransformer, comprising: a first inductive circuit having a firstterminal and a second terminal located side-by-side and symmetrically toa first axis; and a second inductive circuit having a third terminal, afourth terminal and a fifth terminal, wherein the third terminal and thefourth terminal are located symmetrically to the first axis, the firstaxis symmetrically crossing the fifth terminal, wherein the secondinductive circuit comprises a plurality of tracks, wherein tracks of thefirst inductive circuit are located next to and at opposite sides of theplurality of tracks of the second inductive circuit, and wherein atleast one track of the first inductive circuit is narrower than theplurality of tracks of the second inductive circuit.
 16. The baluntransformer of claim 15, wherein the first terminal and the secondterminal are located on a first side of the balun transformer, andwherein the third terminal, the fourth terminal, and the fifth terminalare located on a second side of the balun transformer opposite the firstside.
 17. The balun transformer of claim 15, wherein the fifth terminalis connected to a midpoint of the second inductive circuit.
 18. Thebalun transformer of claim 15, wherein the second inductive circuitcomprises two loops between the third and fourth terminals, and whereinthe first inductive circuit comprises three loops between the firstterminal and the second terminal.
 19. The balun transformer of claim 15,wherein the first inductive circuit has an octagonal shape.
 20. Thebalun transformer of claim 10, wherein the primary inductive circuit hasan octagonal shape.